The present invention relates to a process for the production of an aluminum-containing iron oxide crystallization nucleus with an xcex1-FeOOH crystal structure from FeCl2. This nucleus is suitable as starting material for the production of iron oxide red (xcex1-Fe2O3) via xcex1-FeOOH as intermediate product.
Synthetic iron oxides are normally produced by the Laux process, Penniman process, precipitation process, neutralization process or roasting process (Ullmann""s Encyclopedia of Industrial Chemistry, 5th Ed., 1992, Vol. A20, p. 297 ff). The iron oxides obtained in this way are generally used as pigments.
For the production of finely particulate xcex1-FeOOH (needle width between 5 and 30 nm) two processes are known:
the so-called acid process and
the so-called alkaline process
In the acid process an iron (II) component, such as an iron salt dissolved in water, is used as starting material and an alkaline component, such as an alkali or alkaline earth metal compound dissolved or suspended in water or also ammonia solution, is added thereto while mixing thoroughly. The amount of alkaline component that is added is generally between 15% and 70% of the stoichiometrically required amount. The pH after the addition of the alkaline component is in the weakly acidic range.
After completion of the addition of the alkaline component the reaction mixture is oxidized with an oxidizing agent, such as atmospheric oxygen. The reaction is carried out at temperatures between 20xc2x0 C. and 50xc2x0 C. At significantly higher temperatures there is the danger of the formation of undesirable magnetite. The end point of the reaction can be recognized by a sharp drop in pH and redox potential. After completion of the reaction the properties of the resultant product (generally termed crystallization nucleus) are determined and if appropriate the product is directly processed further into xcex1-FeOOH pigment.
The alkaline process differs from the acid process by the amount of alkaline component that is added. In the alkaline process the amount of added component is at least 120% of the stoichiometrically necessary amount, and is usually significantly greater. The temperatures at which this reaction is carried out may be somewhat higher than the temperatures employed in the acid process, since the danger of the formation of magnetite is not as high in this case.
In the alkaline process relatively long needle-shaped xcex1-FeOOH crystallites are usually obtained with a length to width ratio of 10:1 to 30:1. Since these crystallites furthermore have a very low dendrite content this process is particularly suitable for the production of xcex1-FeOOH as starting product for magnetic tapes.
For the production of xcex1-FeOOH pigments for use in paints and lacquers, the crystallization nuclei produced by the alkaline process cannot be used directly or can only be used to a limited extent since in this process all chromophoric metals present in the Fe component are incorporated. These metals (in particular Mn, Cr, Cu, Ni) significantly impair the color properties and thus restrict the use of crystallization nuclei produced in this way as color pigments.
In order to produce iron oxide yellow pigments an xcex1-FeOOH crystallization nucleus is preferably used and this is then coarsened (built up) in the acid, as a result of which the incorporation of the chromophoric metals is reduced. If it is desired to produce a particularly color-pure iron oxide red (xcex1-Fe2O3) from xcex1-FeOOH by calcination, the xcex1-FeOOH serving as starting material should contain only minor amounts of chromophoric metals. Furthermore, the build-up may take place only at a pH of less than ca. 4 since at higher pH values the chromophoric metals are incorporated in increasing amounts. In addition the particle shape of the xcex1-FeOOH considerably affects the color properties, the viscosity of the coating composition and the need for binders.
In order to achieve a desirable low viscosity in the coating composition and a low binder requirement, short-needle xcex1-FeOOH particles are necessary. These may be produced from long-needle xcex1-FeOOH particles by intensive grinding. A more cost-effective alternative is to produce short-needle xcex1-FeOOH particles directly.
In order to control the particle shape of the xcex1-FeOOH crystallization nucleus and thus the shape of the pigment built up therefrom, so as to obtain a low length to width ratio, modifying additives are necessary. The use of B, Al, Ga, Si, Ge, Sn or Pb as crystallization nucleus modifiers is known from U.S. Pat. No. 4,620,879. This patent specification describes an iron oxide yellow with a particularly low silking index, which is achieved by an appropriate procedure for the pigment build-up and by adding the aforementioned modifiers. However, this patent specification does not state how to prepare an xcex1-FeOOH crystallization nucleus for the production of a particularly good (color-pure) xcex1-Fe2O3 pigment.
An object of the present invention is to provide a process for the simple and inexpensive production of a short-needle xcex1-FeOOH crystallization nucleus according to the precipitation process. This xcex1-FeOOH crystallization nucleus should allow the formation of an (xcex1-FeOOH pigment. From this subsequently isolated xcex1-FeOOH pigment an xcex1-Fe2O3 red pigment should be finally produced by annealing.
This object is achieved by the process according to the invention. If iron (II) chloride is used instead of iron (II) sulfate and 3 to 16 mole % of aluminum, based on Fe is added thereto, then a very finely particulate xcex1-FeOOH with an aspect ratio of 2100 to 3100 is obtained. In the present context the aspect ratio denotes the mathematical product of the BET surface and mean crystallite size that has been determined by X-ray analysis of the 110 reflection of the xcex1-FeOOH.
The invention relates to a process for the production of aluminum-containing iron oxide crystallization nuclei having an xcex1-FeOOH crystal structure with an aspect ratio of 2100 to 3100 by using FeCl2, containing the steps of
a) initially adding an Al component, while stirring, in amounts of 6 to 20 mole % based on total Fe, to an iron II chloride solution with a total Fe content of 20-100 g/l, preferably 40-65 g/l, and a Fe III content of 0.1xe2x88x9210 mole % Fe III (based on total Fe),
b) heating this mixture to a precipitation temperature between 30xc2x0 C. and 60xc2x0 C., preferably between 35 and 50xc2x0 C.,
c) adding a precipitating agent with an active constituent content of 2-10 equivalents per liter, preferably 4-8 equivalents per liter, to the mixture, and the molar ratio Fe+Al to precipitating agent is 20%-80% of the stoichiometric amount, preferably 30% to 60% of the stoichiometric amount,
d) then oxidizing the precipitated suspension with an oxidizing agent at an oxidation rate of 2-50 mole %/hr. of the iron to be oxidized.
The Al-containing xcex1-FeOOH crystallization nucleus obtained after the oxidization may if desired be used without further isolation, after checking the properties, for the production of iron oxide red pigments via xcex1-FeOOH as intermediate.
The following procedure is preferably employed:
Starting chemicals:
FeCl2 solution with an Fe content of 55 g/l Fe, of which 1.5 mole % is Fe III
AlCl3 solution
NaOH solution with an NaOH content of 300 g/l=7.5 equivalents NaOH/l
Al/Fe ratio: 12-13
Ratio Fe+Al/precipitating agent: 30-40%
Reaction conditions:
Temperature: 34xc2x0 C.
Oxidation rate: 30-35 mole % Fe II/hr.
AlCl3 (as aqueous solution) is preferably used as Al component. The use of Si or Ti in the form of their chlorides as crystallization nucleus modifier is also possible, but involves a greater technical expenditure in the production.
Suitable precipitating agents include NaOH, KOH, Na2CO3, K2CO3, Mg(OH)2, MgO, MgCO3, Ca(OH)2, CaO, CaCO3, NH3 or secondary or tertiary aliphatic amines in aqueous solution or as an aqueous slurry. Preferably, NaOH is employed.
Suitable oxidizing agents include atmospheric oxygen, oxygen, ozone, H2O2, chlorine, nitrates of the alkali or alkaline earth metals or NH4NO3. Atmospheric oxygen is preferred.
If the iron II chloride solution that is used contains relatively large amounts, at pH values of less than 4, of chromophoric metals that can be precipitated, then these can be precipitated up to pH 4 by adding an alkaline component to the iron II chloride solution. The solid that is formed can be removed by sedimentation, filtration or centrifugation from the supernatant clear purified solution. In addition to the undesired chromophoric metals, Fe III, which has a significant undesirable influence (formation of black magnetite) on the reaction to form the xcex1-FeOOH crystallization nucleus, is also thereby removed.
The reaction can be carried out batchwise or continuously in stirred vessels, in cascades of stirred vessels, in recycle reactors or in stirrer-less reactors using twin-feed nozzles as mixing devices.
After production of the xcex1-FeOOH crystallization nuclei according to the invention the latter are converted into a pigment, which is effected by a coarsening of the nuclei particles known per se (pigment build-up). Since the xcex1-FeOOH crystallization nuclei according to the invention are however not used as such, it is necessary to describe the pigment build-up and the annealing to form an iron oxide red pigment.
The Al-containing crystallization nucleus produced by the process according to the invention is pumped into a solution of FeCl2 or FeSO4 or another Fe II salt. In this connection, 7-15 moles of Fe II salt in the form of a solution with an Fe content of 30-100 g/l Fe are added per 1 mole of FeOOH in the crystallization nucleus. This suspension is now heated to the reaction temperature, which is between 50xc2x0 C. and 90xc2x0 C. Oxidation and precipitation start simultaneously after the precipitation temperature is reached. As a rule atmospheric oxygen is added via a suitable gassing device and the pH of the suspension is regulated with an alkaline precipitating agent. The pH value is regulated in the range from 2.4 to 4.8. The oxidation rate should be between 0.5 and 8 mole % Fe III/hr.
The most preferred final products are obtained if the following parameters are adjusted during the formation of the pigment:
Ratio xcex1-FeOOH (crystallization nuclei): FeCl2=1:10
Temperature: 60xc2x0 C.
Final pH (reaction): 3,4
Oxidation velocity: 4 mol % Fe/h
Fe-content of the FeCl2: 90-100 g/l
After completion of the reaction (i.e. when all Fe (II) is oxidized) the solid that is formed is separated by filtration. The solid is washed salt-free and can then be dried. Since according to another aspect of the invention an xcex1-Fe2O3 red pigment is to be produced from this xcex1-FeOOH, it is expedient to pass the washed solid directly to a suitable annealing unit.
The annealing of the xcex1-FeOOH pigment conveniently takes place in a continuously operating apparatus. Revolving tubular furnaces, continuous shaft furnaces, fluidized bed reactors, falling shaft furnaces and continuous reheating furnaces are suitable for this purpose. The necessary annealing temperatures (measured in the product) are only between 550xc2x0 C. and 800xc2x0 C. The necessary mean residence times are between 10 and 80 minutes. The xcex1-Fe2O3 pigment that is formed is then preferably subjected to a screening grinding in order to remove oversized material and agglomerates.
The best results are obtained at annealing temperatures of 700xc2x0 C.-800xc2x0 C. at a dwell of 30-60 minutes.
The red pigment that is thus obtained is characterized by a high color purity, virtually isometric particle shape, low oil absorption index and high chemical purity. On account of the sum total of its properties the pigment is particularly suitable for:
use in the paints and lacquers sector
uses as raw material for catalysts
uses in the foodstuffs colorant sector
uses in the paper printing ink sector
uses in the polymer colorant sector
uses as UV stabilizers
uses in the high-quality building materials sector (plaster, etc.)
uses in the dispersion paints sector
This process is particularly economical with respect to the relatively low annealing temperatures, inexpensive raw materials and the high production rate in gel pigment production. Because of the particular reaction conditions and the use of a strictly specified crystallization nucleus it is possible to reliably produce particularly high-quality red pigments that have application technology advantages compared to pigments produced by other methods. Environmentally harmful chemicals are not employed in the production according to the invention of the red pigments.
In a preferred production procedure (use of FeCl2, AlCl3, NaOH and air as starting substances) an almost closed substance circulation is possible by electrolysis of the NaCl formed as byproduct. The sodium hydroxide solution obtained thereby may be directly reused in the process.
The products H2 and Cl2 formed in the alkali chloride electrolysis may be converted into HCl, which in turn then serves for the pickling of steel sheet material. It is not possible at the present time to use this particularly environmentally friendly technology with FeSO4 since the electrolysis of the Na2SO4 does not proceed satisfactorily.
Description of the employed measurement methods
1. Measurement of the BET surface
The BET surface is determined by the so-called 1-point method according to DIN 66131. 90% He and 10% N2 is used as gas mixture, and the measurement is carried out at 77.4 K. Before the measurement the sample is heated for 60 minutes at 140xc2x0 C.
2. X-ray measurement of the crystallite size
The crystallite size is measured in a Philips powder diffractometer.
The 110 reflection is used to determine the crystallite size.
xcex1-iron oxide hydroxide (M(FeOOH)=88.9 g/mole
2.1 Application range
Determination of the crystallite size in goethite in the range from 5 to 100 nm
2.2 Principle
The measurement in goethite is made after X-ray diffractometric irradiation by detecting the reflection. The evaluation is made using silicon as external standard.
2.3 Reagents
Silicon standard for angular calibration (ICDD-No. 27-1402). Philips PW 1062/20
2.4 Equipment
2.4.1 Diffractometer:Philips PW 1800 goniometer
Type: Theta-2 Theta
2.4.2 Sample feed:21 x sample exchanger
2.4.3 Detector: Xe proportional counter tube
2.4.4 Reflection evaluation: X-Pert Software Rev. 1.2 on HP
Vectra VL
2.4.5 Mortar and pestle from Achat
2.4.6 Sample carrier: Philips PW 1811/00 and PW 1811/27
2.5 X-ray diffractometry conditions
2.5.1 X-ray tube: long fine focus, Cu anode, 60 kV, 2200 W
2.5.2 Radiation: CuKxcex1l, xcex=0.154056 nm
2.5.3 Generator: 40 kV, 40 mA
2.5.4 Scan parameters:
2.5.4.1 Scan type: Step Scan
2.5.4.2 Step size: 0.020xc2x0 2Theta
2.5.4.3 Step measurement time: 2.00 sec.
2.5.5 Silicon standard:
2.5.5.1 Starting angle: 27.00xc2x0 2Theta
2.5.5.2 Final angle: 30.00xc2x0 2Theta
2.5.6 Sample:
2.5.6.1 Starting angle: 18.50xc2x0 2Theta
2.5.6.2 Final angle: 23.50xc2x0 2Theta
2.6 Execution
2.6.1 External standard:
2.6.1.1 Insert the silicon standard (2.1) in the sample carrier of the diffractometer and start the measurement program.
2.6.1.2 Measure the maximum and the half width of the silicon reflection with the Miller indices hkl=111 in the 2Theta angular range 27.00xc2x0 to 30.00xc2x0. Print out the peak parameters (Table 1) and optionally the diffractogram.
2.6.2 Measurement in the sample:
2.6.2.1 Grind about 2 g of sample in the Achat mortar (4.5).
2.6.2.2 Add about 1 g of sample to the sample carrier (4.6) of the diffractometer and start the measurement program.
2.6.2.3 Measure the maximum and the integral width of the goethite reflection with the Miller indices hkl=110 in the 2Theta angular range 18.50xc2x0 to 23.50xc2x0. Print out the peak parameters (Table 2) and if necessary the diffractogram.
2.7 Calculations
2.7.1 Enter the integral width (width of broadened profile), the maximum (peak position/xc2x0 2Theta) of the goethite reflection as well as the reflection half width (width of standard profile/FWHM) of the silicon standard in the crystallite size measurement table displayed by the computer (X""Pert software, Rev. 1.2 (Philips Analytical GmbH, Kassel, DE) profile widths). Prepare and print out the evaluation protocol (Table 2).
2.7.2 The crystallite size in the X""Pert program is determined according to the Scherrer equation,       D          (              crystallite        ⁢                  xe2x80x83                ⁢        size            )        ⁢            k      ·      λ                                W          size                ·        cos            ⁢              xe2x80x83            ⁢      θ      
D(crystallite size) Crystallite size in nm
k Form factor of the crystallites=0.9 (mean value from the literature)
xcex Wavelength in mm
Wsize Integral width of the goethite reflectionxe2x80x94reflection half width of the silicon standard
cosxcex8 Maximum of the goethite reflection in xc2x02Theta
3. Measurement of the color values
The color values are measured as described in EP-A 0 911 370.